The load cell described in my copending application Ser. No. 280,772 comprised a beam member having electric resistance strain gages secured to upper and lower surfaces thereof. The strain gages were connected to a Wheatstone bridge circuit to indicate the magnitude of force transmitted to the beam member.
The load cell incorporating electric resistance strain gages offered significant improvements over the devices previously devised. However, under certain operating conditions noise in electrical amplifying systems introduced disrupting signals which introduced error and also established upper and lower limits to the magnitude of the electrical signal which could be accurately measured.
In the employment of electrical resistance strain gages, a grid of fine wire is subjected to the same strain as the surface of the test piece to which it is attached. Since strain is defined as deformation divided by the length in which the deformation occurs, the strain at the surface of a relatively stiff beam is small when the beam is subjected to a light load as compared to the strain when the beam is subjected to heavy loads.
In the design of a load cell employing electrical resistance strain gages in an electrical system to indicate loading on a beam, design of the load cell required careful selection of electrical components having limited operating ranges to provide reasonably accurate indication of force applied to the beam.
In summary, under certain operating conditions, electrical resistance strain gages, attached to a load beam of a load cell, and the electrical circuitry associated therewith have certain inherent characteristics which limit precise measurement of force over a wide range of force applied.